High resolution positional feedback is a requirement of many precision motion systems, in particular motion systems designed to fixture articles for laser processing systems used in the manufacture of electronics. Laser processing systems typically are used to remove material from articles, mark the surface or interior of articles, singulate articles fabricated on a common substrate, or anneal or otherwise condition the article. Articles can be made of metals, such as aluminum or steel, glass or glass like materials or various plastics. Articles can be substrates such as semiconductor wafers or printed wiring boards, devices such as integrated circuits or electro-optical devices, assemblies such as displays or sensors or packaging components such as enclosures.
Some laser processing systems are constructed with motion stages that can fixture more than one article at a time in order to increase system throughput. These systems have motion stages that move articles from positions where they can be loaded or unloaded, to positions for laser processing, and possibly positions for other operations such as further laser processing or inspections. An exemplary laser processing system that performs these functions is the ESI ML5900 Laser Micromachining System, manufactured by Electro Scientific Industries, Portland, Oreg. 97229. FIG. 1 shows a diagram of a typical laser processing system such as the ESI ML5900. These systems have to index articles relatively long distances between positions while maintaining accuracy which is a small fraction of the distance moved. This motion could be rotary, linear, such as a continuous belt, or reciprocating. For example, a rotary stage may move an article the equivalent of several centimeters and require that the article's position be known to within several microns at the end of the motion. This could be accomplished by building the equipment with the requisite mechanical precision to accomplish this task, but the cost and time to operate are both high. The approach typically used is to build the mechanical components to within normal manufacturing tolerances and then instrument the motion stages with encoders to indicate the position of the stage. Typical approaches to the problem involve the use of high quality linear or rotary optical encoders of fine grid pitch or linear interferometers to provide high resolution feedback throughout the motion systems' travel range. Both solutions are well-established, however the costs associated with providing such high resolution feedback throughout the entire travel range is large. FIG. 2 shows a typical prior art rotary stage indexing system.
There are no evident industry solutions to position feedback requirements that combine low resolution-low cost feedback devices with high resolution-high cost devices. Accordingly, there is a continuing need for a method and apparatus for obtaining high resolution positional feedback data from motion stages at costs that are between low resolution-low cost devices and high resolution/high cost devices.